It is an amazing world we are living it :) In Europe nobody is commuting 170 miles per day with a petrol-operated vehicle because burning so much gas is both polluting a lot and also very expensive; that means it's either train or relocating.
If you really want to do it then you need to trade comfort for cost. If you want comfort you should go for a big tourer, if you want to get cheap you need a small engine - 250 or 500 cc top. With my Honda CB 500 I got impressive mileage outside of the city (over 500 kilometers from a 19 liter tank, that's over 60 miles per gallon, I think). The trick is to take it easy on the gas and don't push it over 75 mph. Anyway, look for a faired bike as good wind protection is needed. SV650 is an option, I preffer Honda CBF 600 (injection only, the one on carburetors has lower mileage) and Yamaha Fazer 600.
Another option that I would look for: recent generation of Volkswagen car with diesel engine. Believe it or not, you can drive 100 km with just 3-3.5 liters of diesel, that's up to 90 miles per gallon. I did this over long distances of flat land and I raised to 4 liters per 100 km crossing mountains (and the onboard computer reported near-zero consumption on the entire descent).